Radiosurgery is a non-invasive medical procedure for various kinds of tumors and one of the most effective means for treating local and regional targets such as brain tumors. Instead of a surgical incision, radiosurgery delivers a high dose of high energy photons in radiated beams to destroy the tumor. Radiosurgery is a very efficient method for treating cancers and avoids loss in quality of life compared to other more invasive methods such as surgery or chemotherapy. Since radiated high energy photons can also damage normal cells that are irradiated as the beam passes through a patient to irradiate a tumor, the key of a good radiosurgery plan is to maintain a sharp radiation dose falloff from the high radiation dose regions (high dose regions) inside the tumor to the low radiation dose regions (low dose regions) of nearby healthy structures. The steep radiation falloff rate of dose distribution—known as the “dose falloff rate”—guarantees that normal, healthy tissue and other body parts or structures near the target receive a low dose of radiation while the center of the target or tumor receives a high dose of radiation. Sharper radiation dose falloff will results in better tumor control and less damage to the normal tissue and other body parts surrounding the tumor that are irradiated by the radiation beams.
Focused Beam Geometry:
Currently, most radiosurgeries are performed in a “step-and-shoot” manner and use a number of precisely focused external beams of radiation that are aimed at the target from different directions to increase the dose falloff rate (see FIG. 1). In this technique, as the number of radiation beams increases, the dose falloff rate improves. Therefore, a large number of radiation beams focus on a target to create a high dose region around the target at the point of intersection of the beams. Intuitively, if the number of beams is increased, the contribution of each beam inevitably decreases, resulting in a lower dose to the tissues and structures some distance away from the target. This is because more beams pass through different parts of the body at lower radiation doses but collectively provide the same radiation dose to the target.
However, in these conventional radiation treatments the number of radiation beams is constrained to several hundred beams due to various spatial and physical constraints. For example, in Gamma Knife® radiosurgery, the number of radiation beams is limited to about two hundred beams. Physically, it is not possible to drill a large number of apertures in a fixed size metal screen without eventually causing interference among the beams escaping from the apertures.
For intensity-modulated radiation therapy (IMRT), it is usually not practical to deliver more than a dozen beams due to prolonged treatment time. Even with rotational techniques, such as Tomotherapy, intensity-modulated arc therapy (IMAT), volumetric modulated arc therapy (VMAT), and arc-modulated radiation therapy (AMRT), the maximum number of radiation beams is still limited to a few hundred.
Fundamental Physics Underlying Photon Radiosurgery:
The fundamental physics underlying photon-based radiosurgeries includes high energy photon production and photon interactions with matter.
Generally, high energy photons used in current radiosurgeries are produced either by radioactive decay from Cobalt-60 sources or bremsstrahlung interactions in a linear accelerator. In the linear accelerator, electrons are accelerated in an electric field to a high energy and then collide with a metal target. This generates radiation particles or photons in a bremsstrahlung process. The photons produced from Cobalt-60 are called “γ-ray” or gamma rays whereas the photons produced from a linear accelerator are called “X-ray” or X-rays.
Typically photons produced by different sources are heterogeneous in energy. For example, the energies of γ-rays emitted by Cobalt-60 are 1.17 and 1.33 MeV. The energy spectrum of X-rays from a linear accelerator shows a continuous distribution of energies for the bremsstrahlung photons superimposed by characteristic radiation of discrete energies. The energies of photon beams created by a 6MV accelerator are continuous from 0 to 6 MeV with a large number of photons having energy around 2 MeV. For examples, Gamma Knife®(see FIG. 3) uses γ-rays emitted from radioactive Cobalt-60 sources to irradiate tumors, while Cyberknife® (see FIG. 4), which is essentially a linear accelerator carried by a robotic arm, uses X-rays to irradiate tumors.
When photons pass through matter, they interact in one of three ways: Photoelectric effect, Compton effect and Pair production. For radiosurgery, the predominant interaction is the Compton effect, where the incident photons collide elastically with orbit electrons. During this elastic collision, energy is imparted from the incident photons to orbiting electrons and sets off a chain of reactions. These electrons know as secondary electrons, as they travel through matter, produce ionization and excitation along their path. On a cellular level, these ionizations damage DNA and cause cell death in the body.
Important Beam Characteristics for Treatment Planning:
A percent depth dose curve relates the absorbed dose deposited by a radiation beam into a medium. FIG. 2(a) shows the percent depth dose curve of Cobalt-60 with an 80 cm Source Surface Distance (SSD). Two parameters of a radiation beam are its Tissue Maximum Ratio (TMR) and Off Center Ratio (OCR). TMR is defined as the ratio of the dose at a given point in phantom to the dose at the same point at the reference depth of maximum dose. OCR is the ratio of the absorbed dose at a given off-axis point relative to the dose at the central axis at the same depth. FIG. 2(b) shows the TMR of Cobalt-60 and a 6MV accelerator. FIG. 2(c) shows the OCR of a 6MV accelerator.